The STAR isobar data of \(^{96}\) Ru+\(^{96}\) Ru and \(^{96}\) Zr+\(^{96}\) Zr collisions at \(\sqrt{s_{\mathrm {NN}}}= 200\)  GeV show that ratios of observables (\(R_{\mathcal {O}}\) ) such as the multiplicity distribution, \(p(N_{\mathrm {ch}})\) , and the harmonic flow, \(v_n\) , deviate from unity, when presented as a function of centrality, \(c\)  [1]}. These deviations have been attributed to the differences in the shape and radial profiles between \(^{96}\) Ru and \(^{96}\) Zr nuclei. In addition, the ratios \(R_{\mathcal {O}}(x)\)  depend on the choice of the event activity variable \(x\) , which could be either \(N_{\mathrm {ch}}\)  or centrality. We estimate the difference \(\Delta R\)  between these two choices, based on the published \(p(N_{\mathrm {ch}})\) , as well as those from a multiphase transport (AMPT) model with varied nuclear structure parameters: nuclear radius (\(R_0\) ), surface diffuseness (\(a_0\) ), quadrupole deformation (\(\beta _2\) ), and octupole deformation (\(\beta _3\) ). In contrary to \(R_{v_n}(c)\) , \(R_{v_n}(N_{\mathrm {ch}})\)  is nearly independent of the analysis approaches, suggesting that nonflow effects are better controlled by \(N_{\mathrm {ch}}\)  than \(c\) . The ratios of observables sensitive to the chiral magnetic effect (CME) are also much closer to unity for \(x=N_{\mathrm {ch}}\)  than \(x=c\) , indicating that the ratios calculated at the same \(N_{\mathrm {ch}}\)  provide a better baseline for the non-CME background. According to the AMPT results, the dominant parameter for \(\Delta R\)  is \(a_0\) , while \(R_0\)  and \(\beta _n\)  are only important in central collisions. The published \(p(N_{\mathrm {ch}})\)  is also used to estimate \(\Delta R_{\left\langle p_{\mathrm {T}}\right\rangle }\)  for mean transverse momentum, which is non-negligible compared with \(R_{\left\langle p_{\mathrm {T}}\right\rangle }-1\) .

25.75.Gz, 25.75.Ld, 25.75.-1
